1. Field of the Invention
This invention relates to analysing audio signals.
2. Description of the Prior Art
Several techniques have been proposed for showing, on a visual display, various technical features of an audio signal.
One previously proposed technique is the so-called "voice print". A typical "voice print" represents a monophonic sound by a two dimensional image on a computer screen, paper print or cathode ray tube display.
A horizontal axis is used to represent time, with the earliest on the left and the latest (or most recent) time on the right. A vertical axis is used to represent frequency, with lowest frequencies at the bottom and highest at the top.
It is usual for the vertical axis to be on a logarithmic scale, i.e. equal vertical distances representing octave differences in frequency. The intensity of the image at each point represents the intensity of the sound at the appropriate frequency and time. The amplitude to intensity mapping used is usually logarithmic, i.e. changes in the decibel (dB) value correspond to changes in intensity. Depending upon the type of display used (paper printout or screen display) louder sounds may be represented by a darker or lighter image element. Images may be either a static "snap-shot" of a sound over a number of seconds, or may be continuously generated in real time either onto a roll or paper, or scrolling across a screen.
Voice print images have been in use for over 40 years, maybe much longer. However, they use essentially a monophonic technique. If the technique is to be used with a stereo signal, then either a separate voice print has to be produced for each channel, or the two channels have to be combined so as to produce a single audio signal whose time-dependent intensity can then be mapped onto the voice print. Neither of these solutions then gives any indication of the relative phase of the stereo channels.
Another previously proposed technique which allows the relative phase of a stereo pair to be displayed graphically is the so-called "phase-scope" display.
In this device the left and right parts of a stereophonic signal are displayed on an oscilloscope screen such that the left signal displaces the spot upwards along an axis from the bottom right comer to the top left comer of the display and the right signal displaces the spot upwards along an axis from the lower left comer to the top right comer of the display. Given this arrangement the "phase-spot" displayed on a phase scope may differentiate between the following signals:
SILENCE: Stationary spot in centre of screen.
LEFT only: Line from bottom right to top left.
RIGHT only: Line from bottom left to top right.
FRONT.sup.1 : Vertical line
BACK.sup.2 : Horizontal line
ORTHOGONAL.sup.3 : Central elliptical/circular display.
RANDOM.sup.4 : "Ball of wool" central display. FNT .sup.1 Left and right of equal amplitude and in phase FNT .sup.2 Left and right of equal amplitude and out of phase (here, it is appreciated that the "back" is not strictly correct, but it is used throughout this description as a useful term to distinguish out-of-phase signals from in-phase or "front" signals). FNT .sup.3 Similar sinusoidal left and right signals with 90.degree. phase difference FNT .sup.4 Uncorrelated left and right signals
However, although the phase scope provides useful graphical information about the relative phases of the left and right channels, the "phase scope" also suffers from a number of disadvantages:
In contrast to the voice print display, the phase scope display is transient and requires that the operator keep an eye on it whenever anything interesting happens to the audio signal under test.
Also, the "phase scope" display works on the aggregate stereo signal, which is usually composed of the outputs of many instruments which have different directional characteristics. This makes it difficult to distinguish the directional information in one signal in the presence of all the others.